Technology update

Physicists at the Niels Bohr Institute at the University of Copenhagen in Denmark and Microsoft's Station Q Copenhagen laboratory have produced and detected Majorana "zero modes" with unprecedented clarity in a quantum-dot hybrid-nanowire system. These modes, which are formed by delocalized states of electrons with topological properties might be used as qubits in fault-tolerant quantum computers of the future.

Majorana particles, which are their own antiparticles, were first predicted by the Italian physicist Ettore Majorana in 1937. They obey non-Abelian statistics, which means that quantum information encoded in them would not decohere (be destroyed) easily.

Physicists have not yet seen isolated Majorana particles in experiments, but some collective excitations of electrons in solids happily have the same properties as Majorana particles. Indeed, such Majorana quasiparticles have already been spotted in several systems including semiconducting nanowires coated with a superconducting layer. When cooled to near absolute zero temperatures, superconducting electrons can exist within the semiconductor wires. An electron in this wire becomes entangled with electrons on either side of it to create a continuous chain of entangled electrons along the entire length of the wire.

At both ends of this chain there are electrons that are only entangled with one electron, and these can be considered as “half” an electron. Known as Majorana modes, they together form a Majorana quasiparticle. Quantum information stored in such a quasiparticle should be distributed between both ends of the nanowire, which means that it would be protected from being destroyed by external noise. To destroy it you would have to act on both ends of the wire at the same time – an unlikely scenario.

Observing Majorana-bound states

Now, a team of physicists led by Charles Marcus of the Niels Bohr Institute at the University of Copenhagen has observed these Majorana bound states in a hybrid indium arsenide (InAs) nanowire using a quantum dot at the end of the wire as a “spectrometer”. The researchers created nanowires that were 5 to 10 microns long using molecular beam epitaxy and then used low-temperature epitaxial growth to coat the wires with aluminium (Al), which is a superconductor at ultralow temperatures. These techniques produce a pure, ordered interface between the semiconductor and superconductor, something that is crucial for such experiments.

Next, a transene-D Al etch was used to selectively remove the Al from the end of the wire, which was then coated with titanium/gold (Ti/Au) electrodes to form a normal (that is non-superconducting) metal lead that was used to measure currents in the set up. A quantum dot was formed in the bare InAs segment between the Ti/Au contact and epitaxial Al shell.

Topological phase emerges

When a magnetic field is applied to the system, the researchers observe a topological phase emerging in which pairs of “Andreev”-bound states move to zero energy and merge, so forming Majorana-bound states, just as theory predicts. “This result also backs up work done by our colleagues at Delft University in the Netherlands in 2012 and really gives us a clear view of what is going on here,” says Marcus.

“The idea in our experiments was to tunnel normal electrons into the Majorana wires to measure where there were bound states,” he adds. “The dots simply provide a tall sharp barrier for the tunnelling.”

The main point here is that we now have more confidence when identifying the features we have observed as being associated with Majorana zero modes (MZMs), he tells nanotechweb.org. “It was important for us to feel confident that these were really MZMs.”

Moving on to the next (more difficult) stage

“Of course, in physics, as in all experimental sciences, we don’t prove things, we just see evidence. But now, the evidence we have is strong enough to motivate us to move on to the next (more difficult) stage of our experiments without the feeling that we are wasting our time.”

So what are the next steps? “We would now like to use our nanowire to create qubits and indeed show that they are protected from decoherence,” says Marcus. “Our recipes are described in a number of publications that you can consult here, here and here.